Aquaporins and cell migration

Abstract

Aquaporin (AQP) water channels are expressed primarily in cell plasma membranes. In this paper, we review recent evidence that AQPs facilitate cell migration. AQP-dependent cell migration has been found in a variety of cell types in vitro and in mice in vivo. AQP1 deletion reduces endothelial cell migration, limiting tumor angiogenesis and growth. AQP4 deletion slows the migration of reactive astrocytes, impairing glial scarring after brain stab injury. AQP1-expressing tumor cells have enhanced metastatic potential and local infiltration. Impaired cell migration has also been seen in AQP1-deficient proximal tubule epithelial cells, and AQP3-deficient corneal epithelial cells, enterocytes, and skin keratinocytes. The mechanisms by which AQPs enhance cell migration are under investigation. We propose that, as a consequence of actin polymerization/depolymerization and transmembrane ionic fluxes, the cytoplasm adjacent to the leading edge of migrating cells undergoes rapid changes in osmolality. AQPs could thus facilitate osmotic water flow across the plasma membrane in cell protrusions that form during migration. AQP-dependent cell migration has potentially broad implications in angiogenesis, tumor metastasis, wound healing, glial scarring, and other events requiring rapid, directed cell movement. AQP inhibitors may thus have therapeutic potential in modulating these events, such as slowing tumor growth and spread, and reducing glial scarring after injury to allow neuronal regeneration.

Fig. 1

Impaired tumor growth and endothelial cell migration in AQP1 null mice. a Left. Tumor in an AQP1^+/+ vs an AQP1^−/− mouse, 2 weeks after subcutaneous injection of 10⁶ B16F10 melanoma cells. Right. Tumor growth data (ten mice per group, mean ± SEM, P<0.001). b Left. Wound healing of cultured endothelial cells (initial wound edge, blue; after 24 h, red). Right. Wound edge speed (n=4 per group, mean ± SEM, *P<0.01). c Tracks of six migrating CHO cells expressing AQP1 vs six non-AQP expressing control CHO cells, tracked for 4 h. Initial cell position indicated by arrow. d AQP1 protein (green) polarization to lamellipodia (arrows) in a migrating CHO cell. For more details, see [31]

Fig. 2

Slowed migration of AQP4 null astrocytes. a Left. Representative photos of Boyden chamber migration assay showing AQP4^+/+ and AQP4^−/− astrocytes (blue) after scraping off the non-migrated cells. In these experiments, astrocytes were plated on the top chamber of poly-L-lysine coated transwells and were allowed to migrate through 8-μm pores using 10% FBS as chemoattractant. Bar = 100 μm. Right. Summary of migration experiments (15 AQP4^+/+ vs 13 AQP4^−/− transwells, mean ± SEM, *P<0.001). b Phase contrast micrographs (top) and outline (bottom) of the leading end of a migrating AQP4^+/+ and AQP4^−/− astrocyte in the in vitro wound assay. Arrows show direction of migration. Numbers are fractal dimensions (larger number denotes more irregular cell membrane). Bar = 10 μm. c AQP4 protein (green) polarization to the front end of migrating astrocytes in the in vitro wound assay. Arrow shows direction of migration. d Left. Stab injury/cell injection model of astrocyte migration in mouse brain. Two days before cell injection, a stab was created as shown. Cultured AQP4^+/+ and AQP4^−/− astrocytes were fluorescently labeled and injected as indicated. Right. Locations of migrating fluorescently stained AQP4^+/+ cells (green) and AQP4^−/− cells (orange). The x-axis is shown as percentage of distance between injection and stab injury sites (x_rel). Data obtained from 15 different mice. e High-magnification fluorescence micrographs of a migrating AQP4^+/+ and AQP4^−/− cell. Bar = 5 μm. For further details, see [1, 32]

Fig. 3

AQP1 expression increases lung metastasis after tail vein injection of tumor cells. a Left and middle. Hematoxylin and eosin staining of paraformaldehye-fixed paraffin-embedded sections of mouse lung tissue at 14 days after tail vein injection of 10⁶ control or AQP1-expressing 4T1 breast tumor cells. Tumor metastases indicated by arrows. Right. AQP1 immunohistochemistry showing labeling (brown) of alveoli/vessels in both micrographs, with AQP1 also in tumor cells in the lower micrograph. b Data summary showing number of metastases per lung, area of tumor colonies, and alveolar wall thickness within 50 μm of metastases (five mice per group, mean ± SEM, *P<0.02). See [13] for further details

Fig. 4

Proposed mechanisms of AQP involvement in cell migration. a Schematic showing changes in cell shape, which take place as cells migrate through the extracellular space. AQP expression may facilitate the transmembrane water movements that mediate rapid changes in cell volume. b i) Actin depolymerization and ionic influx increase osmolality at the front end of the cell. ii) Water influx across the cell membrane increases local hydrostatic pressure causing cell membrane expansion, which forms a protrusion. AQP polarization to the front end of the cell facilitates water flow into the cell. iii) Actin re-polymerizes to stabilize the emerging protrusion. See text for further explanations